1. Field of the Invention
This invention relates to an apparatus for recirculating combustion exhaust gases to the burner region of a combustion device for lowering combustion temperatures and reducing the formation of nitrogen oxides (NO.sub.x).
2. Description of the Prior Art
It is generally known that, if formation of nitrogen oxides (NO.sub.x) is to be kept at a low level, high temperatures during combustion should be avoided if air is used as the oxidizing medium. It is also known that exhaust gas recirculation is an effective method of lowering peak temperatures during the combustion process and thus minimizing the formation of NO.sub.x. This principle has been utilized in the design of engines depending upon the combustion process for their energy source, including hot gas engines utilizing the Stirling cycle. The U.S. Pat. Nos. 3,456,438 to R. J. Meijer et al. and 3,546,876 to H. Fokker et al. show Stirling cycle applications utilizing combustion gas recirculation for controlling combustion temperatures and NO.sub.x formation.
The major drawbacks of exhaust gas recirculation are the costs of the necessary special components to achieve recirculation, the decrease in overall efficiency because of the additional hydraulic flow losses occuring in the recirculation apparatus, and the maintenance costs for the additional apparatus. In the known devices for utilizing exhaust gas recirculation in conjunction with hot-gas engine operation, the exhaust gases are mixed with the inlet air prior to the air being admitted to the combustion region. This mixing can be accomplished after the inlet air has exited the preheater apparatus in order to reduce the volume of gases flowing through the preheater and thus minimize hydraulic losses in that component. However, the hydraulic losses which occur in these known exhaust recirculation devices are still large and degrade the overall performance of the engines.
One of the major shortcomings of existing prior art exhaust recirculation devices is that the recirculation is accomplished outside the general vicinity of the burner region of the combustion device, necessitating additional conduits and external mixing devices such as a fan or an externally mounted ejector. The present invention eliminates the need to externally mount these components and thereby achieves a significant reduction in the length of conduits needed to carry the recirculated exhaust gases and thereby effects a reduction in the consequent hydraulic flow losses.